A. Field of the Invention
The present invention relates to a noncontaminating thrusting separation system and more particularly in preferred embodiments to the longitudinal separation joint portion of an explosively operated linear separation system that may be used for separating payload fairing protective structures from launch vehicles during the flight of a rocket, missile or other vehicle.
B. Background of the Related Art
Linear explosive separation systems have found many uses in space launch vehicle applications as shown in FIG. 7. For example, such systems are used in longitudinal fairing split-line joints 50, to allow the jettison of fairing structures 51 to uncover the protected spacecraft payload 52. Explosive separation systems are also used in circumferential fairing base joints 54 and interstage joints 56. Typical linear separation systems in use include the thrusting joint, the expanding-tube (X-Tube) double fracture joint, the expanding-tube rivet shear joint and the flexible linear shaped charge (FLSC) or mild detonating cord (MDC) severance joint.
One prior art thrusting joint separation system, as taught in U.S. Pat. No. 3,362,290 to Carr et al., is typically used in payload fairing split-line separation applications having moderate to low longitudinal structural joint shear requirements. This thrusting joint uses the controlled flow of hot gases generated by the detonation of MDC to inflate a flexible bellows. The action of the expanding gases in the inflatable bellows creates a force used to shear joint retaining rivets and provide thrusting energy to jettison the fairing shell structures. The system is noncontaminating, in that the products of detonation do not leave the bellows, and eliminates the need for additional separation thrust mechanisms. However, the joint is heavy and has poor load carrying capability prior to separation. The poor load carrying capability is a direct result of the limited shear force that the pressurized bellows can generate. This limited shear force requires that a limited number of relatively weak rivets be used to join the fairing shell structures prior to separation.
Expanding-tube double fracture separation systems are typically used in large diameter, highly loaded circumferential interstage or payload fairing base separation applications. These double fracture type joints are also used in payload fairing longitudinal split-line separation applications requiring high shear release forces. The double fracture joints are lightweight with high load carrying capability. However, as the tube expansion is predominately in the plane of separation, a non-thrusting separation is achieved. Thrust, if required, must be obtained through the incorporation of spring actuators or other thrusting mechanisms. Two expanding-tube double-fracture type joints are described in U.S. Pat No. 3,698,281 by Brandt et al. and U.S. Pat No. 4,685,376 by Noel et al.
Expanding-tube rivet shear type separation joints can be used in highly loaded circumferential or longitudinal joints where separation velocity requirements are minimal or the mass to be jettisoned is relatively small. An exemplary prior art expanding-tube joint 60 is presented in FIG. 8. The high force, short stroke expanding-tube device is well suited for shearing retention rivets. As the tube expansion is in the direction of the separating joint halves, a small thrust is produced.
FLSC and MDC severance type joints have been used for the linear separation applications shown in FIG. 7, for example, but they are contaminating. That is, the debris and other products of the detonated explosive are dispersed into the environment, where they may interfere with the operation of the payload or the launch vehicle, especially where delicate instruments are involved. The joints are lightweight and capable of carrying high structural loads, but the shock and debris generated by the unshielded cord at the separation event make them unattractive for many launch vehicle applications. A typical FLSC-type interstage separation joint is taught by U.S. Pat. No. 3,185,090 to Weber and a typical MDC-type missile stage separator joint is taught by U.S. Pat. No. 3,139,031 to Schroter et al.
To provide additional background information on the present invention, a brief historical discussion is presented on mild detonating cords, expanding-tube sheathed mild detonating cords, thin walled steel expanding-tubes and inflatable bellows thrusting joint systems. Rivet shear capabilities and joint weights of several thrusting joints are compared and a summary is presented.
Linear explosive cords were originally developed to transfer an explosive stimulus from one point to another. Such products are often employed in blasting. For the last 30 years, detonating cords have found space-age applications, e.g., in missile stage and fairing systems. The linear explosive used in these systems is typically a mild detonating cord (MDC). The explosives used in the core of these systems and the present invention can be any of the wide variety of explosives. Examples of such explosives are taught by U.S. Pat. No. 3,311,056 to G. A. Noddin. Chief among these are PETN, RDX, HMX, HNDS, DIPAM, and HNS as presented in Table 1. The explosive core of the MDC is typically contained within a suitable cover material. For example, PETN is typically contained in a waterproof textile cover; whereas, the other explosive cores are typically contained in a drawn, soft metal exterior cover, such as lead, aluminum or silver. Upon detonation of the explosive core this outer cover fragments, thereby contaminating the immediate vicinity with the by-products of the explosion and the cover material. For explosive core loads of useful interest to the present invention (5 to 25 grains per foot), these MDCs (core plus cover) range from 0.090 to 0.120 inches in diameter. The detonation propagation velocity for these MDCs ranges from 8,300 meters per second for PETN to 6,800 meters per second for HNS. These high velocities are advantageous in that they result in near simultaneous operation of all portions of a linear joint system.
TABLE 1 ______________________________________ Typical Mild Detonating Cord Explosive Compounds ______________________________________ PETN pentaerythritol tetranitrate RDX cyclotrimethylenetrinitramine HMX cyclotetramethylenetetranitramine HND hexanitrodiphenylsulfone DIPAM dipicramid HNS hexanitrostilbene ______________________________________
Environmental requirements for noncontaminating and low shock producing linear separation systems have led to developments, such as the non-rupturing detonating cord taught in U.S. Pat. No. 3,311,056 to G. A. Noddin and the similar expanding tube technology taught in U.S. Pat. No. 3,373,686 to Blain et al. These inventions use the radial expansion of a sheathed MDC to perform a structural severance or separation function without contamination of the adjoining area. Both of these inventions have a radially expanding tube assembly consisting of an elastomer outer tube, and MDC, which is coaxially located within the sheath. The sheath is designed to position and protect the MDC, enhance the transmission of the detonation shock and pressure energy, and, in some applications, contain the products of detonation. As taught in the Blain '686 patent, the sheath used in the expanding-tube device is typically constructed of an elastomeric material having sufficient radial elasticity to perform useful work against an adjoining body, and further having sufficient tensile strength to prevent rupturing of the sheath material when radially expanded by the generated force of detonation of the explosive core. Detonation of the MDC produces a shock wave and an associated pressure increase which causes an immediate uniform radial expansion of the tube. The force generated by the radial expansion of the tube is sufficient to do work such as to sever an adjoining structure or to shear rivets.
Tube assemblies formed of two or more differing materials--for example, a thin walled stainless steel tube housing an inner elastomer sheath--were also taught by the Blain '686 patent. Another example of a thin walled expanding-tube structure was taught in U.S. Pat. No. 3,486,410 to Drexelius et al., which employed a flattened malleable expanding-tube containing a linear explosive charge. The pressure generated from the explosive charge caused expansion of the flattened tube is capable of performing useful work such as severing an adjoining structure. This thin-walled expanding-tube technique was further developed in the expanding-tube double-fracture type joints as taught in U.S. Pat. No. 3,698,281 to Brandt et al. and optimized in U.S. Pat. No. 4,685,376 to Noel et al. In these inventions the expanding-tube device comprises a partially flattened metal tube with a coaxial linear explosive(s) running the full length of the tube. The tube ends are sealed and fitted with booster end tip detonator devices. When the linear explosive is activated, the resulting shock and internal pressure increase causes the tube to expand, whereupon a pair of doublers or a clevis type arrangement enclosing the tube is fractured and separated along a weakened section underlying a notch or groove that extends longitudinally along the doubler joint. The tube expansion is predominately in the plane of separation; therefore, a non-thrusting separation is produced.
Typically, in thin walled expanding-tube devices, the linear explosive is encased in a support material such as silicone rubber to maximize energy transfer to the tube wall. The sealed tube assembly imparts displacement energy without releasing debris or gases from the device itself. The small displacements and high forces generated by expanding-tube devices are suited to fracturing adjoining structures as used in the Brandt '281 and Noel '376 patents, or releasing shear pin attachments found in tongue-and-groove type linear separation systems as shown in FIG. 8. Referring to FIG. 8, a typical tongue-and-groove type joint has a female member 62, and a male member 64, an expanding-tube device 65 and a shearable connector 69. The expanding-tube device comprised a partially flattened metal tube 66 in which is disposed an explosive cord 68 wrapped by an elastomeric sheath 67. This expanding-tube assembly is placed between the female member 62 and the male member 64. Connector 69 then joins the members. Detonation of the explosive cord 68 causes expansion of tube 66, which exerts forces that causes the connector 69 to fail in shear. When used in a rivet-shear tongue-and-groove arrangement, the tube expansion is in the direction of the separating joint halves, generating some usable thrust energy in addition to the high rivet-shear separation forces.
One of the advantages of such expanding-tube systems is the relatively large amount of force that they can produce. This force may be used to sever structures or to shear rivets. A disadvantage of this type of system is that the force is exerted over a relatively small stroke distance. Therefore, at most, only a small amount of thrust can be imparted to formerly connected components using expanding-tube devices.
The Carr '290 patent teaches another type of linear explosive separation system. This noncontaminating separation system utilizes a linear piston-cylinder combination with an explosive cord contained within and running the full length of the joint. The piston and cylinder, which are held together by a row of retaining rivets, form a chamber which extends the length of the joint. The linear explosive is contained within two concentric stainless steel attenuator tubes which are in turn confined within a thin walled flexible bellows. This tube-bellows assembly is contained in the chamber formed by the piston-cylinder. The concentric tubes, consisting of a smaller tube inside a larger tube, serve to control and contain the shock of detonation and control the flow of hot gases produced by detonation of the explosive into the bellows through gas metering and directing openings in the tubes. This gas flow metering and directing is necessary to prevent perforation of the bellows material by the concentrated flow of fast moving hot particles from the exploding MDC. When the linear explosive is detonated, the rapidly expanding gases pass through the vent holes in the attenuator tubes into the bellows. The action of the expanding gases in the bellows creates a force which reacts against the piston-cylinder combination within the chamber, shearing the retaining rivets and producing thrust to separate the joint, which may be coupled to fairing half-shell structures.
In the thrusting joint as described above, all products of detonation are fully contained within the bellows, resulting in a noncontaminating separation. The thrusting joint separation system described above thus performs two functions during the separation event: Shear rivets to release the piston-cylinder joint halves, and thrust to provide a separation velocity to the fairing half-shells. The energy required to shear rivets is usually calculated on a force-per-foot basis. For example, typical longitudinal joint shear structural requirements for payload fairings range, for example, from 5,000 to 20,000 pounds per foot (lbs/ft). Shear forces generated by hot gases metered into the flexible bellows are dependent upon the bellows pressure and the piston area. On a unit length (per foot) basis, the piston area is a function of the piston width; therefore, available shear force (lbs/ft)=bellows pressure (psi).times.piston width (in).times.12 (in/ft). However, shear force generation can be limited by several factors: MDC core loading limitations, attenuator tube confinement capability, detonation shock limitations, peak gas pressures obtainable, bellows operating pressure limits, piston width limits, or overall separation system weight limits.
While thrusting joints have the advantage of a relatively long stroke distance that is of benefit in providing thrust to push apart separated pieces, they have the disadvantage of providing relatively weak shear forces with which to cause an initial separation of connected pieces, e.g., to shear rivets or other connectors.
Applications exist for lightweight, medium to small sized fairings that have joint shear requirements in the 10,000 to 20,000 lbs/ft range, for example. Prior art thrusting joint systems are not capable of providing adequate shear forces without becoming too large and heavy for these applications.
The various types of linear separation joints heretofore known in the art have advantages along with inherent disadvantages that limit their usefulness, as shown in the following table:
TABLE 2 ______________________________________ Joint Joint Contamina- Joint Type Strength Weight ting? Thrust ______________________________________ Thrusting Joint Low High No High Expanding-Tube High Low No None Double Fracture Expanding-Tube High Low No Low Rivet Shear FLSC/MDC High Low Yes None Severance ______________________________________
As can be seen, none of the existing systems provides high thrust combined with the capability of severing a high strength joint. It is desirable that such a joint be provided, preferably having a low or medium joint weight.